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2004 2500 6.0 Alternator Whining

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I just purchased a 2004 GMC 2500 with a 6.0. The truck has 130k miles on it. The guy I bought it from told me the alternator needed to be replaced because it had a whine that would come through the radio. He had replaced the radio because he thought it was bad...but the whine did not go away. I replaced the alternator, but had the same issue. I then took the original alternator to be bench tested, and it tested good. I put it back in, but the alternator has a whine when accelerating and it comes through the radio. 

To troubleshoot, I unplugged the wiring clip while running...and it makes zero whine under the hood. If I reconnect the the wiring, it starts making an audible whine that sounds just like the sound coming through the speakers over whatever music may be playing.


It is driving me crazy...has anyone else had this issue?

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The problem lies with whats called a Ground loop. The radio, for lack of a better term, is using the speakers as a ground.

 Pull the radio. If you used an adapter pigtail, make sure you Solder the power and ground leads.

 Check the ground wire size, make sure its same as power. Check the ground wire ( truck off, key out) coming to the radio, to truck ground with a ohm meter. I turn on the beep on my meter, so I don't have to look at the resistance.

 Probably a NEW ground wire from the chassis of the radio, to a GOOD truck ground, ( possibly one of the 2 cigar /12 volt outlets below) will do the trick. Good luck!

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Thanks for the advice! I pulled the radio and ran a new ground to the chassis, it was a 12gauge wire. 


Here is the odd thing, the loudest part of the whining is coming from the actual alternator. 

It completely goes away if I unplug the electrical connector while running...and instantly comes back. 

I checked all of the body grounds, as well. 


Its the strangest thing I've ever seen!

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I can turn the radio up and hear the hum.

I can then turn the radio off,  and go pop the hood and the same noise is coming form the alternator. 
If I unplug the connector on the alternator while running, it goes quiet. 
The whining comes back when the electrical connector is plugged back in (while running).
The alternator doesnt make any bearing noise and is fully bench tested. 
It even made the noise with the new alternator. 

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Adding a capacitor ( noise suppressor) is an old trick that used to work well, and may just fix this.

( credit to Pizza)

 The Body control module ( all cab functions) GROUND is UNDER the truck, on the DS, right under the DS door

on the CAB mount Extension facing the REAR of the truck. These ground wires need to be removed and cleaned well,

and frame cleaned. After put some waterproof GREASE on it..

 Did you do this ground?

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